Differential vulnerability to relapse into heroin versus cocaine-seeking as a function of setting.

RATIONALE: Previous studies have shown that the effect of setting on drug-taking is substance specific in both humans and rats. In particular, we have shown that when the setting of drug self-administration (SA) coincides with the home environment of the rats (resident rats), the rats tend to prefer heroin to cocaine. The opposite was found in nonresident rats, for which the SA chambers represented a distinct environment.
OBJECTIVES: The aim of the present study was to investigate the influence of setting on the ability of different doses of cocaine and heroin to prime cocaine- versus heroin-seeking in rats that had been trained to self-administer both drugs and had then undergone an extinction procedure.
METHODS: Resident (N = 62) and nonresident (N = 63) rats with double-lumen intra-jugular catheters were trained to self-administer cocaine (400 μg/kg/infusion) and heroin (25 μg/kg/infusion) on alternate days for 10 consecutive daily sessions (3 h each). After the extinction phase, independent groups of rats were given a noncontingent intravenous infusion of heroin (25, 50, or 100 μg/kg) or cocaine (400, 800, or 1600 μg/kg), and drug-seeking was quantified by counting nonreinforced lever presses.
RESULTS: All resident and nonresident rats acquired heroin and cocaine SA. However, cocaine primings reinstated cocaine-seeking only in nonresident rats, whereas heroin primings reinstated heroin-seeking only in resident rats.
CONCLUSIONS: We report here that the susceptibility to relapse into drug-seeking behavior is drug-specific and setting-specific, confirming the crucial role played by drug, set, and setting interactions in drug addiction.